1. Field of the Invention
The present invention relates to a fuel injection control device with improved exhaust gas.
2. Description of the Related Art
Conventionally, in diesel engines, it has been common to perform combustion with fuel injected in the vicinity of compression top dead center, where a high temperature and high pressure are attained inside the cylinder (inside the combustion chamber).
The injected fuel is mixed with intake air to form a mixture, this mixture is ignited so that a flame is formed, and combustion is continued by supplying subsequent injected fuel to this flame. In this combustion system, ignition begins during the injection of the fuel; this will be referred to as “ordinary combustion” in the present specification.
In recent years, a new combustion system has been proposed in which NOx(Nitrogen Oxide) and smoke can be greatly decreased without causing any deterioration in fuel consumption by setting the fuel injection timing earlier than compression top dead center so that the ignition retardation period is lengthened, thus promoting sufficient mixing of the fuel and intake air (for example, see Japanese Patent Application Laid-Open No. 2001-20784 and Japanese Patent Application Laid-Open No. 2002-327638).
In concrete terms, fuel injection is performed during the period extending from the intake stroke to the compression stroke prior to compression top dead center, and ignition begins after a specified pre-mixing period has elapsed following the completion of this fuel injection. In this combustion system, the ignition retardation period is long, so that the mixture is sufficiently rarefied and uniformly mixed; accordingly, the local combustion temperature drops, so that the amount of NOx(Nitrogen Oxide) that is emitted decreases. Furthermore, since combustion in a state of insufficient air can be locally avoided, smoke is also suppressed. Combustion in which ignition thus begins following the completion of fuel injection, and the injection system used to realize this combustion, are referred to as “early pre-mixing combustion” and “early pre-mixing injection” in the present specification.
Thus, this early pre-mixing combustion (injection) is effective in improving the exhaust gas; however, control of the ignition period of the fuel is difficult, so that such combustion has been difficult to realize. In other words, in the case of the ordinary combustion described above, since the fuel is ignited during the injection of the fuel, the ignition timing can be controlled relatively easily by controlling the injection timing; however, in the case of early pre-mixing combustion, the ignition timing cannot be sufficiently controlled merely by controlling the fuel injection timing.
Furthermore, although details will be described in the section titled “Description of the Preferred Embodiments”, the present inventors discovered that in the case of early pre-mixing combustion, there is a danger not only of a failure to obtain an exhaust gas cleansing effect, but also of a deterioration in HC (Hydrocarbon) emission amounts, fuel consumption, unless the period extending from the time of completion of fuel injection to the beginning of ignition (this period is referred to as the “pre-mixing period” in the present specification) is controlled to an appropriate range.
In other words, appropriate control of the ignition timing and pre-mixing period of the fuel is important for realizing early pre-mixing combustion.